All eukaryotes investigated so far contain multiple copies of ubiquitin genes, most of which are arranged in fusions coding for either polyubiquitin or ubiquitin-ribosomal protein constructs; the former are normally under the control of a heat shock promoter. Giardia lamblia, an intestinal parasite, is the most primitive eukaryote known to date. We have investigated the arrangement and expression of ubiquitin genes in this organism by Southern and Northern blotting. Our data strongly suggest that G. lamblia contains just one ubiquitin gene, which consists of a single copy of the coding sequence and the expression of which is not enhanced by heat shock. By pulsed-field gel electrophoresis we localized this gene on the largest of the five giardial chromosomes. These data imply that the ubiquitin system in Giardia has probably been trapped at an original stage.